1. Field of the Invention
The present invention relates to a thin amorphous alloy film or covering film having a good workability and able to be used as a soft magnetic material, and a process for the preparation thereof.
2. Description of the Related Art
In an amorphous alloy, the arrangement of atoms is irregular and the amorphous alloy lacks the long periodicity of and differs in structural characteristic from a crystalline alloy in the non-existence of a crystal grain boundary or a lattice defect, and therefore, the amorphous alloy has superior magnetic characteristics and the application of the amorphous alloy as a low hysteresis loss material or a high permeability material is under consideration. For example, the application of an Fe-based amorphous alloy to a core of a transformer is considered because this amorphous alloy has a high saturation flux density and a low hysteresis loss, and accordingly, when this alloy is used, the hysteresis loss is much lower than in a transformer having a conventional core composed of a silicon steel sheet, and good characteristics can be obtained. Furthermore, a Co-based amorphous alloy is characterized in that the coercive force is low over a broad frequency band region, and this amorphous alloy can be effectively used as a magnetic core material for a magnetic amplifier.
The most popular process for the preparation of an amorphous alloy is a rapid quenching process, whereby a melted metal is quickly guided onto a cooled rotary drum and rapidly quenched at a cooling rate of about 10.sup.5 to about 10.sup.6 deg/sec, to solidify the melt without allowing time for a crystallization thereof, whereby an amorphous alloy is formed. In general, however, the amorphous alloy formed by this rapid quenching process is limited to a thickness of scores of .mu.m, because undulations are formed on the surface when the melted metal is guided onto the drum, and thus it is difficult to obtain a thin product. As a means of eliminating these undulations, a process was proposed in which a melted metal is placed in contact with a cooled plate surface under a high vacuum, and has been reported that a thin film having a thickness of about 6 .mu.m can be obtained by this process [M. Yagi et al, J. Appl. Phys., 64, 6050 (1988)]. If such a thin film can be obtained, the eddy current can be reduced and a material having a lower hysteresis loss and a higher permeability can be obtained. In this process, however, a high vacuum is necessary and a large-scale apparatus must be used, and thus it is impossible to continuously prepare a foil. Therefore, the process is not industrially utilizable.
Other known processes for the preparation of an amorphous alloy are a sputtering process, an ion plating process, and a vacuum evaporation deposition process, and although thin films can be obtained by these processes, the speed of the growth of the amorphous alloy and the productivity are low, and the preparation apparatus is expensive and requires a large installation area. Accordingly, these processes are not suitable for mass production. Furthermore, according to these processes, it is difficult to recover a product in a form suitable for a wide range of applications, such as a tape-shaped film or a metal foil, and therefore, the product can be used only when deposited on a substrate having an intended form.
Recently, attempts have been made to utilize an electric plating or electroless plating process in which an amorphous alloy is obtained by depositing a metal ion or metalloid ion contained in a solution by electrolytic deposition or chemical reduction (see Japanese Unexamined Patent Publication No. 52-140403 and Japanese Unexamined Patent Publication No. 55-164092).
In the field of soft magnetic materials, a material having a low loss, a high conversion efficiency, and superior high-frequency characteristics are required, and to obtain these characteristics, the magneto-striction, one of the magnetic characteristic parameters, must be reduced as much as possible. The amplitude of the magneto-striction has a close relationship to the composition of the alloy, and it is well-known that the compositions of ferromagnetic elements giving a magneto-striction of zero are Fe/Co (6/94), Fe/Ni (18/82) and Co/Ni (46/54). Among these compositions, the Fe/Co system having a high saturation flux density is considered an excellent soft magnetic material, and based on this consideration, a process for the deposition by the plating method of an amorphous alloy composed mainly of Co has been developed. For example, Japanese Examined Patent Publication No. 63-10235 discloses a process in which an electric current having a density of 5 to 20 A/dm.sup.2 is applied to an acidic plating bath having a pH value of 1.2 to 2.2, which comprises, as main ingredients, about 1/6 to about 3 moles/l of a divalent cobalt ion and about 0.03 to about 0.3 mole/l of hypophosphorus acid and/or a hypophosphite, to effect the electroplating, whereby an amorphous alloy composed mainly of cobalt and phosphorus is deposited.
Currently, improvements of the characteristics in the high-frequency region are especially required for a soft magnetic material to be used as a magnetic core or the like, and as described above, if a high-frequency exciting current flows in a magnetic layer, usually the quantity of an eddy current flowing in the magnetic layer is increased, resulting in increase of the loss. Accordingly, as the frequency is increased, the permeability is reduced and the coercive force is increased. The eddy current is in inverse proportion to the resistivity of the magnetic layer; i.e., in the same material, the eddy current is in proportion to the thickness of the magnetic layer, and a thinner magnetic layer has better characteristics.
For an industrial product, the ability to be easily and precisely formed into an intended shape and size is important not only for increasing the productivity and reducing costs but also for improved performances. Accordingly, a tape-shaped, sheet-shaped or foil-shaped material having a good surface smoothness has superior general-purpose properties. For example, when applied to a magnetic head, a magnetic material is produced in a -shaped form by pressing, and the shaped pieces are laminated, welded and integrally joined by using a laser beam or the like. This application can be greatly facilitated if the magnetic material is in the form of a tape, sheet or foil.
Note, Japanese Examined Patent Publication No. 63-10235 mentioned above does not disclose a specific process for forming an amorphous alloy foil. Namely, in the process disclosed in Japanese Examined Patent Publication No. 63-10235, phosphorus added to the alloy for stabilizing the amorphous state of the alloy is supplied from hypophosphorous acid and/or a hypophosphite contained in the plating solution, but when hypophosphorous acid and/or a hypophosphite is used as the phosphorus-supplying agent, the phosphorus concentration range at which a good film-forming state can be maintained is very narrow and the process is defective in that the operation range is restricted. Moreover, this process does not have a satisfactory film-forming property, and accordingly, it is difficult to obtain a very thin film. Therefore, it is substantially impossible to obtain a very thin film having a reduced eddy current loss and superior magnetic characteristics.